Much progress in the electronics industry comes from circuit size reduction. This is most directly accomplished by running photolithographic processes at ever-shorter wavelengths of light. Processes using 193 nanometer (nm) light are undergoing commercialization while 157 nm wavelength light is under development as a next generation candidate.
In immersion photolithography (Switkes et al, J. Vac. Sci. Technol. B, 19 (6), 2353 6, November/December 2001) an optical source and a target surface are immersed in a highly transparent high refractive index liquid. As shown by Switkes et al Microlithography World, May 2003, pp. 4ff, higher resolution in photolithography can be achieved at a given wavelength of incident light when a high refractive index transmission medium is employed. Realization of the potential benefits of this technology is dependent upon identifying high refractive index liquids having high transparency in the VUV/DUV and excellent photochemical stability.
All known organic materials absorb to some extent UV radiation of 193 nm. The issue is whether liquids can be found that are sufficiently transparent to be practical. Short chain alkanes H(CH2)nH and short chain fluorocarbons F(CF2)nF are relatively transparent compared to their longer-chain homologues at 193 nm, as disclosed, for example, in B. A. Lombos et al, Chemical Physics Letters, 1, 42 (1967); G. Belanger et al, Chemical Physics Letters, 3(8), 649(1969); and K. Seki et al, Phys. Scripta, 41, 167(1990).
Although fluorinated species are known to be transparent to UV radiation of some useful wavelengths, and fluorination can decrease absorbance, fluorinated molecules can exhibit low refractive indices about, e.g., about 1.3, which is undesirably low immersion lithography at 193 nm.
Gaseous materials may be highly transparent, can have lower than desired refractive indices. Liquids with low boiling points similarly can have an undesirably low refractive index and require a pressure vessel for containment.
Crews, P.; Rodriguez, J.; Jaspars, M., Organic Structure Analysis. ed.; Oxford University Press: 1998; p 353 discloses 200 nm as the short wavelength cut-off for transparency in cyclohexane.